1. Technical Field
The present invention relates in general to production trees for subsea oil wells and, in particular, to an improved system, method and apparatus for a modular production tree assembly for reducing weight during transfer of the tree from a barge to a rig.
2. Description of the Related Art
A conventional subsea wellhead assembly includes a wellhead housing that supports one or more casing hangers located at upper ends of strings of casing extending into the well. A production tree or “tree” is landed on the wellhead for controlling the production of well fluids. The tree usually carries a choke and valves to control the flow and sensors to monitor the flow.
Trees are cumbersome and very heavy. Prior to the installation of a tree on the sea floor, it must be delivered to the offshore rig at sea and hoisted onto the rig platform. The size and weight of trees makes it difficult to lift them from a delivery barge onto the rig. In order to lift larger trees onto the rig, they must be broken down into their various components and then reassembled and retested prior to deployment.
Deployment may require a flowbase to be guided onto the wellhead and then the tree guided to the well and flow base to align the flowline connections. In shallow water this is traditionally done with wires, such as a 5-leg sling set made up to the tree frame. This adds weight to the design as the frame must be suitably strengthened. In deep water a guideline-less approach is normally used, such as a lift cap with a single leg sling that is clamped or dogged to the tree mandrel.
In addition, some form of a guide funnel is normally required between the tree and the flowbase and/or wellhead. This configuration also adds weight and complexity to the overall assembly. A choke bridge may be used to connect a tree to a template, or be integral with the tree which may require an additional pipe spool and hydraulic connection to make up to a flowbase in the case of satellite trees. This again requires additional connections adding weight and complexity. Although these solutions are workable, an improved design that overcomes the limitations and expense of prior art designs would be desirable.